In the state of the art, so-called oscillatory forks are known, with which the fill level of a medium in a container can be monitored. In such case, a mechanically oscillatable unit in the form an oscillatory fork is excited to resonant oscillations by a piezoelectric transducer element. For example, two part piezo drivers with at least one transmitting and one receiver piezo (e.g. DE 39 31 453 C1) or one-piece piezo drivers with only one piezoelectric element (e.g. DE 197 20 519 A1) serves for oscillation excitement or reception.
In the case of the one piezo technology, one piezo element serves both as transmitter and receiver, i.e. the piezo, via which the mechanical system is excited to its resonance frequency also senses the oscillations of the mechanical system. The exciter signal, with which the piezo is supplied, is, in such a case, for example, a rectangular electrical alternating voltage. On the edges of the rectangular signal, the polarity of the piezo capacitance is reversed, whereby charging- and discharging electrical currents arise. Additionally, an electrical current corresponding to the mechanical movements flows during the pulses. The resulting electrical current can be converted to a voltage via a measuring resistor and evaluated.
Since the charging- and discharging electrical currents of the piezo capacitance contain no information concerning the mechanical oscillation, there are undesired side effects, which are suppressed in the state of the art (see DE 197 20 519 A1), for example, by means of a reference capacitor. This happens e.g. in a measuring bridge in which a compensation capacitor is calibrated so that it corresponds to the piezo capacitance. If during operation, i.e. while measuring, the transducer element—i.e. the piezo capacitor and the compensation capacitor are supplied with the same exciter signal, in the case of equal capacitance, equal behavior in the sensed voltage is shown in both by the capacitive fractions in reference to the reverse charging events, or the reverse charging peaks (or the reverse charging electrical currents). The voltage sensed from the transducer unit comes from the received signal, which, thus involves both the charging/discharge curves and the actual wanted signal representing the mechanical oscillations of the oscillatable unit. If the two capacitor voltages are subtracted from one another (or also added in the case of a preceding inversion), then from the received signal there remains only the actual wanted signal, which carries information concerning the mechanical oscillations of the mechanically oscillatable unit (here the oscillatory fork.).
A problem exists in the case of the application of a compensation capacitor in that such shows a different aging and also temperature behavior than the piezoelectric transducer element. Most often, materials are used for capacitors, which have a relatively low temperature coefficient of the dielectric constant. In contrast therewith, for example, piezo materials based on LZT (lead zirconate titanate) possess a very high temperature coefficient of the dielectric constant. Thus there is a clearly different temperature behavior of the respective capacitances. The dielectric properties of LZT materials also change with time (aging). Therefore, a detuning of the compensation can occur both via temperature and also via time, in the state of the art.